This is what happens when you have too much time on your hands...

I was thinking this same thing yesterday, no it was Saturday, while taking pictures of my daughter and her prom date. I can't remember why I was thinking about this, but that doesn't matter.
I did a search and found that this question has been asked quite a few times but no answers have been given.

 

I've wondered at it for years. I've never bought into the percentage points, for example take a nitrous car. My car made 348 RWHP on motor, and 456 on a 100 shot. You can't tell me that the same transmission, driveshaft, and rear end take 52.2 HP to turn on motor and 68.4 HP to turn when you hit the button LOL.. I would imagine there is an approximate number that it takes to turn everything but it would take a LOT of research to find out what.. and somehow you would have to take into effect the differences in dyno's since it would be pretty hard to find the same brand engine and chassis dyno's in the same place..

 

I agree with you but only partially. I DO believe the fixed number goes up a bit, but by far less amount than a fixed %. I say this because i'm simply applying logic on this one:

if you push 300bhp thru a fixed drivetrain, there might be 40bhp lost, but as you try to shove more and more BHP thru that same drivetrain (all other things constant), the amount of loss via heat and drivetrain capability must go up by some unknown variable. But what is that variable.... we may never know... LOL.

so it all comes down to the most important question: Does blacksnake make 611bhp or 580... or something in between... haha :salute: life is good if these are my issues....

if someone knows, please chime in...

 

i guess the only real way to tell is to dyno the car and then dyno the engine. but thats alot of work

 

Quote from autoblog:

With the new 600-horsepower 2008 Dodge Vipers starting to hit the streets, it was only a matter of time before someone put one on a dyno to see what kind of juice the snakes are sending to the rear wheels. That someone, in this case, is longtime Viper tuner Hennessey, and their new car made 546 horses and 516 lb-ft of torque in stock trim. According to DragTimes, if you factor in a 13% drivetrain loss, the new Viper is actually making around 615 horses. Not shabby, to say the least. Hennessey dynoed the car a second time after making some exhaust mods and was able to raise those numbers to 574.95 rwhp and 537 lb-ft. Head over to DragTimes to see the graph. We'll take one in green, please.


Another Quote from autoblog: http://www.autoblog.com/2008/09/24/corvette-zr1-dynod-at-535-rwhp/
So, how does it fare on the rollers? Two separate 'Vettes were tested, and the average output was about 535 horsepower at the rear wheels. After factoring in the normal 15-percent drivetrain losses for a rear-wheel drive car, that's pretty darn close to the claimed 638.

so I guess its between .13 and .15..... sigh. Still doesn't seem right, but it is what the industry says it is...

 

I'm really surprised that Ford didn't do a bunch of this testing back in, oh, say... 1999? They had a really good reason to LOL.

 

I found this info from an archived post (going years back):


***These questions have been long standing debates among car enthusiasts, so I recently emailed DynoJet and asked these exact questions. Below are the responses I received from the DynoJet representative. ***

QUESTION:
A car having 3:27 rear end gears is placed on a DynoJet chassis dyno and has a dyno performed. Then this same car is taken off the DynoJet chassis dyno and has the rear end gears changed to 4:10. The car is then placed back on the DynoJet chassis dyno and another dyno is performed. Will the second dyno show a loss of horsepower caused by the 3:27 to 4:10 gear change?

DynoJet Technician ANSWER:
Yes
EXPLANATION:
"The 4:10 gear will show less horsepower than the 3:27. The reason is due to rate of acceleration changes. The rate of acceleration is quicker with the 4:10 because of torque multiplication being increased to the rear wheel. The horsepower will show less because the targeted RPM is met before the horsepower has a chance to overcome the rotational mass (dyno, drive line, etc.) or moment of inertia in speed. Because the speed is decreased and the RPM is met faster, the horsepower never has a chance to catch up with itself, so to speak. The overall ratio of 1:1 will always produce the most horsepower on the chassis dyno. Having said this, a similar problem can occur with horsepower loss when the rear gear is too high. The horsepower is being absorbed in just trying to keep the rotational mass spinning. The more HP the car produces, the more loss the car will show on the dyno. For example, if you have a car with 150 rwhp, and go from 3.27 to 4.10 gear, you may only experience 3-5 rwhp reported loss on the dynojet. If you have 300rwhp, the losses (under the same scenario) may be in the 5-7hp range. Please keep in mind that your engine's horsepower never changes but what gets to the dyno or drive surface does. ." If you have any further questions please don't hesitate to ask. Thank you


QUESTION:
A car is placed on a DynoJet chassis dyno and has a dyno performed with 45-PSI tire pressure. Then the same car is immediately placed back on the DynoJet chassis dyno with 35 PSI tire pressure and has another dyno performed. Will the difference in tire pressure from the two dyno runs affect my dyno results, and if so how?
DynoJet Technician ANSWER:
Yes
EXPLANATION:
"Yes, it can. Unfortunately the tire becomes different in form due to different tire pressures. The lower the pressure, the more the tire becomes deformed which causes heat, added friction, and some oscillation. These three things all contribute to not only less horsepower but also contribute to inconsistency in the runs and eventually a ruined tire. If the tire has the correct tire pressure, then the tire is a lot more reliable for repeatability. The tire is also less likely to deform and cause unwanted heat and added friction thereby making the drive train work more efficient. If we have too much air in the tire, then the only thing we really have to worry about is the tire failing. The best results are to put the tire pressure at the manufacturers recommendation on the sidewall of the tire. I know that seems to be a little by the book but doing it that way is the easiest to remember but it is also necessary when you want to compare your runs each time you dyno your car so that you can eliminate one more factor in the repeatability game. Some people think that lowering the tire pressure will provide more traction. This may be true on the street or strip but not true on the dyno. The knurling on the drums have twice the co-efficient slip factor as a paved surface and therefore we never have to worry about traction on the dyno. The only thing you will achieve by lowering the tire pressure on the dyno is lower and inconsistent horsepower."

 

So just to throw and engineering spin on things. I'd suggest that your drive train losses come from the standard F=u*n formula for friction. So your friction force of two gears rubbing on each other or a shaft/bearing will all change dynamically with the load. The friction force = the coefficient of friction multiplied by the normal(90° to the surface) load. So once you've overcome the static friction of all the part interfaces, any interface between parts under a load that is directly(linearly) related to the input force (torque from the motor) will cause a friction force that also relates to the torque produced by the motor in a linear fashion.

Keep in mind that you guys are assuming the motor can accelerate at all rates with the same torque...which isn't true. Every engine reaches a rate of acceleration that no mater how little a load on it, it can't accelerate any faster.

 

Ok then.... now that you've shown off to us poor, slow, blue collar guys; how about providing something useful like a ballpark answer to the question I posed...

 

I know this is old, but ill throw in my thoughts.

Parasitic losses arent a fixed % based on max hp, but a percent of RPM.

I think, at very low RPM, the losses are marginal, the higher you spin the machinery, the more friction is encountered, and the higher the percentage goes.

It has nothing to do with horsepower.

So if your engine is making XXX hp at a given RPM, and you modify it and make more hp at the same rpm, you losses are the same as before.

However, if you shift your powerband and rev higher to achieve the max hp, then the losses are greater due to friction.

TLDR;

Friction losses are a percentage of RPM and not HP

WHatever that is, idk, and it varies wildly based on the machinery being spun

 

Newton's 3rd Law in action.

 

Exactly

 

I had to go looking for this.

Engine Dyno:
"The carbureted combination ran clean and posted 401.2 at 6,100 rpm, and peak torque checked in with 391.1 lb-ft at 4,700 rpm. "

Chassis Dyno:
"It produced 346 hp at 5,900 rpm and 343 lb-ft of torque at 4,700 rpm."

Result:
"This equates to a drivetrain loss of 13 percent-about 57 hp and 72 torque. "

Installing A 331-Stroker Kit In A Small-Block - Stock-Block Stroke Swap Part 4 - Muscle Mustang & Fast Fords Magazine

In our part 4 installment of assembling our small-block 331ci engine, we conclude the buildup by installing a 331-stroker kit into the block. Read on for more details along with photos only in Muscle Mustang & Fast Fords

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My simple mind thinks that 600hp will push gears together harder and create more fiction and hp loss.

 

Think about it like this...

WIthout the engine running, remove spark plugs so there is no compression.

Now, grab the wheels and spin them. That amount of friction and effort is what is lost, essentially.

It doesnt matter if the engine, while running, makes 50 hp or 500 hp.

For drivetrain specifically, youd have to decouple the tranny, but you get the point.

Now, no matter the HP the engine makes, it doesnt change how hard it it to spin your drivetrain.

What DOES affect it is RPM, because with RPM the friction of bearings, seals, rotational mass and oil increases.

 

Post 14 was with a C4 automatic. So that's a bit more loss than through a manual transmission. The rough estimation of losing around 15% is reasonable. Loss through a C4, AOD, 4R70W, T5, TKO, T56 will all be slightly different. Even the R&P will effetc the loss. But it's all kind of moot anyway. How much gets translated to motion is what matters.